Research On Removal Mechanism Of Cr(Ⅵ) From Water By Biochar Supported Sulfide-modified Zero-valent Iron

The common valence states of chromium(Cr)are trivalent chromium(Cr(Ⅲ))and hexavalent chromium(Cr(Ⅵ)),among which the toxicity of Cr(VI)is～100 times higher than that of Cr(Ⅲ).Therefore,it is of great significance to investigate the reduction and immobilization of Cr(Ⅲ).Zero-valent iron(ZVI)with large surface area and high reactivity can remove Cr(VI)through mechanisms such as reduction,adsorption,and co-precipitation,thus is widely used in the remediation of wastewater polluted by Cr(Ⅵ).However,the magnetic property of ZVI can result in its easy agglomeration,and it is easy to be passivated with water and oxygen to form passivated layer,which reduces the electron transfer and reducing ability of ZVI.This paper aims to modify ZVI to improve the removal performance of Cr(VI)by sulfuration and biochar surpport.Scanning electron microscopy(SEM),X-ray diffraction(XRD),and Raman spectroscopy(Raman)were used to characterize the properties of the material before and after modification.The electron transport capacity of the modified materials was analyzed by electrochemical method and the reaction mechanism was elucidated.The main research contents and results of this paper are as follows:(1)In order to synthesize sulfide-modified zero-valent iron(S-ZVI)with better removal performance towards Cr(Ⅵ),different sulfur sources,synthesis methods,and the molar ratio of S to Fe were used to prepare S-ZVI material,and these materials were used to remove Cr(Ⅵ)in water at different pH.The results showed that the S-ZVI material prepared by the post synthesis method with Na2S2O3 as the sulfur source exhibited an obvious lamellar structure,which had the best removal performance on Cr(VI).When the molar ratio of S to Fe increases from 0 to 2,the removal capacity of Cr(VI)increases,and then decreases with the further increase of the molar ratio of S to Fe.When the S to Fe molar ratio is 0.2,the removal amount of Cr(Ⅵ)is up to 29.97 mg/g,which is slightly lower than that of 35.96 mg/g when the S/Fe molar ratio is 2.(2)In order to explore the enhancing effect of sulfurization on the removal of Cr(Ⅵ)by ZVI,batch adsorption experiments were used to explore the influence of environmental factors on the removal performance of Cr(VI)by S-ZVI and ZVI.The isotherm experiments showed that the removal of Cr(Ⅵ)by ZVI and S-ZVI can be fitted with Langmuir model,that is,the adsorption of Cr(Ⅵ)onto the two materials is mainly a monolayer adsorption process.The kinetic experiments showed that the removal process of Cr(VI)by the two materials can be better fitted by the pesedo-second-order kinetic model,that is.the adsorption of Cr(Ⅵ)by ZVI and S-ZVI materials is mainly chemical adsorption,and their maximum adsorption capacity were 12.29 and 31.48 mg/g,respectively.The experimental results showed that the modified S-ZVI material significantly improves the removal capacity of Cr(Ⅵ).Moreover,the magnetic field strength of the modified material barely decrease.The electrochemical and X-ray photoelectron spectroscopy(XPS)analysis indicated that the removal mechanism of S-ZVI for Cr(Ⅵ)includes S(-Ⅱ)and Fe(Ⅱ)reduction on the surface,electrostatic adsorption,and the formation of FeCr2S4 and Cr2S3 co-precipitation.The Tafel corrosion curve and electrochemical impedance spectroscopy(EIS)Nyquist curve showed that the modification of sulfuration significantly improved the electron transport capacity,the reactivity of ZVI,and the removal ability of Cr(Ⅵ).Low pH value is beneficial to the removal of Cr(Ⅵ)by ZVI and S-ZVI,but S-ZVI still has a better removal ability of Cr(Ⅵ)under alkaline conditions,indicating the broad pH range of application of S-ZVI.The removal ability of Cr(Ⅵ)by S-ZVI increased with the increase of temperature.The presence of humic acid(HA)in water partly inhibits the removal of Cr(Ⅵ)by S-ZVI.(3)The method of biochar(BC)as support is proposed to avoid easy agglomeration of S-ZVI,biochar supported S-ZVI composite material(S-ZVI/BC)is optimized through orthogonal test,and the material with the best performance of removing Cr(VI)is screened out through the Cr(Ⅵ)removal experiment in solution.The results showed that the pyrolysis temperature and the feedstock used to prepare biochar are the key factors to determine the Cr(Ⅵ)removal ability of S-ZVI/BC composites.When the pyrolysis temperature is 700 0C,the selected biomass is pine,and the Fe/C ratio is 0.2,the synthesized material has the best removal performance for Cr(Ⅵ).This can be attributable to better dispersion,higher specific surface area,and better electron transport.The removal mechanisms of Cr(Ⅵ)by S-ZVI/BC mainly include reduction,adsorption,and co-precipitation.In addition,S-ZVI/BC material has strong magnetic properties,which can realize the rapid solid-liquid separation of adsorbent.(4)In order to clarify the effects of pyrolysis temperature on the specific surface area,electron transport capacity,and Cr(Ⅵ)removal ability of S-ZVI/BC composites,pine biochar(PBC)at different temperatures were prepared by pyrolysis.PBCs at different temperatures was used to support S-ZVI to synthesize S-ZVI/PBCs.The composites were characterized and used in the Cr(VI)removal experiment.The results showed that with the increase of pyrolysis temperature,the crystallinity and specific surface area of S-ZVI/PBC composites increase,the graphitization degree increases,and the electron transfer rate increases.The removal capacity of S-ZVI(S-ZVI/PBC700)composites during pyrolysis at 700℃ is 36.01 mg/g,which is twice than that of S-ZVI/PBC300 composites(16.05 mg/g).The desorption experiments showed that reduction is the main mechanism of Cr(VI)removal by composites.The adsorption of Cr(VI)by three kinds of composites at different temperatures can be better fitted by the Langmuir model,which indicates that the adsorption of Cr(VI)is mainly a monolayer adsorption process.(5)In order to clarify the influence of feedstocktypes on the specific surface area,electron transport capacity,and Cr(VI)removal ability of S-ZVI/BC composites,biochar(BC)and S-ZVI/BC700 composites with different biomass feedstocks,including pine(PBC),straw(SBC),and cow manure(WBC)were prepared,characterized in a series of ways,and used for Cr(VI)removal experiments.The results showed that S-ZVI/PBC700 composite has the best crystallinity,the highest specific surface area,and graphitization degree,and the highest electron transfer rate.The adsorption experiments showed that the removal capacity of S-ZVI/PBC700 composite for Cr(VI)was the highest,which was 36.01 mg/g,and the removal capacity of S-ZVI/WBC700 composite for Cr(VI)was the lowest,which was 10.39 mg/g.Reduction is the main mechanism of Cr(VI)removal by composite materials.Langmuir model can better fit the adsorption process of Cr(VI)by the composites synthesized from three different feedstocks,indicating that the adsorption process is mainly a monolayer adsorption.